Abstract
Graphene oxide (GO) is a promising membrane system for chemical separation applications due to its 2-D nanofluidics properties and an ability to control interplanar spacing for selectivity. The permeance of water, methanol (MeOH) and isopropyl alcohol (IPA) through 5 µm thick membranes was found to be 0.38 ± 0.15, 0.33 ± 0.16 and 0.42 ± 0.31 LMH/bar (liter/m2·h·bar), respectively. Interestingly, the permeance of a water–alcohol mixture was found to be dramatically lower (~0.01 LMH/bar) than any of its components. Upon removing the solvent mixture, the transmembrane flux of the pure solvent was recovered to near the original permeance. The interlayer space of a dried GO membrane was found to be 8.52 Å, which increased to 12.19 Å. 13.26 Å and 16.20 Å upon addition of water, MeOH and IPA. A decrease in d-space, about 2 Å, was consistently observed when adding alcohol to water wetted GO membrane and an optical color change and reduction in permeance. A newly proposed mechanism of a partial reduction of GO through a catalytic reaction with the water–alcohol mixture is consistent with experimental observations.
Highlights
Graphene oxide (GO) is a promising membrane system for applications in chemical separations due to enhanced 2-D nanofluidics properties and an ability to control interplanar spacing for size-based chemical exclusion
Dramatic flow rate enhancements of 3–4 orders of magnitude have been experimentally seen through carbon nanotube (CNT) membranes with the atomically smooth surface of the graphitic planes making the tube walls [1,2]
Molecular dynamics simulations show that in addition to atomic smoothness [3], water molecules can be structured to a single-chain conformation connected by two hydrogen bonds, giving flow rates greater than aquaporin water channels [4]
Summary
Graphene oxide (GO) is a promising membrane system for applications in chemical separations due to enhanced 2-D nanofluidics properties and an ability to control interplanar spacing for size-based chemical exclusion. The fabrication of mechanically robust membranes with 2-D channels and high porosity over large surface areas remains a challenge and is required for systematic chemical transport and selectivity studies. The slip-casting process was systematically optimized for GO membrane fabrication to improve the microstructure by shear-aligning GO flakes Such membranes were found with water permeance as high as 71 LMH/bar (10.65 μm × LMH/bar), which is over 7 times the permeance of common filtered GO membranes [24]. Free-standing GO powder was Membranes 2021, 11, 317 demonstrated to convert benzylic alcohols to corresponding aldehyde and ketone at the cost of GO being partially reduced [39] Other reactions, such as ring-opening of epoxides, can be catalyzed by GO with a good conversion rate and product selectivity [40]. The reduction process of the GO membrane can be reversed with the addition of oxidant to restore permeance
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